A true example of a coward, One who often backs out of things and rarely shows they have any balls; you can also refer to someone with no backbone as a pile of goop (if any human didn't really have a backbone, they would simply be a pile of goop)

ME: Hey, can you go upstairs and get my keys?
brother: NO I CANT I'LL BE LATE FOR MY HOT DATE!!!
ME: Aww, you have no fucking backbone